The present invention relates to a method and apparatus for evaluating the wind-induced fatigue performance of mechanically attached single-ply membrane roof covers.
Known ultimate load, static test methods and apparatus do not address the cyclic nature of wind loads. Although these tests are adequate in preventing non-fatigue failures of properly installed roof systems, they cannot determine the likelihood of the failure of a roof membrane due to fatigue. Mechanical securement of single-ply membranes is fast becoming one of the more popular installation methods due to its lower installation cost. In this installation method, insulation is secured to a structural roof deck with a minimum number of fasteners, usually one fastener for every four to eight square feet of insulation. A single-ply roof membrane is rolled out over the insulation and attached, through the insulation, to the roof deck at discrete points using mechanical fasteners. Wind-induced uplift pressure is transferred directly from the membrane through the fasteners to the structural deck. Field experience with this roofing system has revealed that fatigue failure modes of fastener twist-out, steel deck fatigue and membrane fatigue do not occur in the known static wind uplift tests.
Other apparatuses are known which evaluate the fatigue of fasteners. In one apparatus, a motordriven cam oscillates a lever which is connected, through an air cylinder and chain, to a membrane secured by a fastener. A test on the apparatus is typically run with the same load applied to a membrane sample, at the same angle, regardless of the material of the membrane. Under actual conditions, a given wind applies loads to different roof cover materials at different angles. The common load and angle applied by the known apparatus for all materials results in the simulation of different wind loads for different materials. Furthermore, with respect to the apparatus itself, it loads one side of the fastener while maintaining a zero load on the other side. This approach is overly severe in that it simulates the membrane on one side of a row of fasteners being inflated by the wind while the other side of the fasteners has no load applied. The apparatus uses pneumatics to regulate the applied loads and uses the concept of an unbalanced load to simulate ballooning of different portions of the roof cover during cyclic winds.
Another known apparatus for evaluating the fatigue of fasteners includes a horizontal platen to which the roof deck, insulation and fastened membrane are attached. The membrane has loose ends attached to arms of the apparatus, which move horizontally. The platen moves vertically at a frequency eight times that of the arm. These different frequencies allow a wave-like action which closely simulates the ripple effect of the membrane. However, because the geometry of the platen and arms is not adjustable, the same motion is applied to all types of membranes. Furthermore, it is not known if a given force during the cycle of the apparatus is applied at the angle at which forces would be exerted under actual conditions. Moreover, the actual forces imparted to the sample in the apparatus are not known.
A drawback for both fastener fatigue apparatuses described above is the fact that only one fastener is used in the test, a relatively narrow strip of membrane is fastened, and the contribution and effect of adjacent fasteners is not considered.